The hammond organ. Not sure who did it first but the sound of one of these plugged into an overdriven amp became the trademark of quite a few of the bands of this era. The first time I got into it was with this album which I got turned onto by one of my sister's guy friends. The other thing with Heep was the unique vocals which would later become a staple with a lot of the NWBHV bands that would surface towards the end of the 70s.
Heep's next two albums were their peak but this one came first for me. This is one of those that listening to it now it's easy to go "meh". But when it first hit the scene it was pretty damn groundbreaking.
Uriah Heep - Look at Yourself
1971

The slide guitar on this blew my mind at the time! So freaking heavy.